1. Field of the Invention
The present invention relates to a remaining battery capacity measuring device, and more particularly, to a remaining battery capacity measuring device which is capable of measuring the remaining capacity of a secondary battery with high accuracy and which is suited to be packaged together with the secondary battery.
2. Description of the Related Art
Chargeable secondary batteries, such as nickel-hydrogen storage batteries, lithium ion batteries, etc., have recently spread to a marked degree, and are now widely used as power sources for various electronic apparatuses including personal computers. These days, moreover, various attempts are made to develop so-called battery packs in which a secondary battery is packaged together with protection circuits for the secondary battery and a measuring device for measuring the remaining battery capacity.
The remaining battery capacity measuring device generally comprises a current measuring circuit for measuring charging and discharging currents flowing through the secondary battery on the basis of a voltage across a shunt resistor for current detection that is connected in series with the secondary battery. The remaining battery capacity is determined by a microprocessor which cumulates the charging and discharging currents that are detected by the current measuring circuit. The current measuring circuit of the battery capacity measuring device is required to detect the charging and discharging currents of the secondary battery with high accuracy. Further, the battery capacity measuring device including the current measuring circuit and operated by electric power supplied from the secondary battery is required to minimize its power consumption.
However, the conventional current measuring circuit comprised of discrete circuit components is relatively high in power consumption. Further, improving the current measurement accuracy of the current measuring circuit makes the configuration of the circuit complicated. It is very difficult, therefore, to package the battery capacity measuring device, including the current measuring circuit, together with the secondary battery to form a battery pack.
Conventionally, current fuses and the like are used for the protection of the secondary battery against overcurrent. However, the current level for the fusing of the current fuses is insecure, and besides, the fusing time varies depending on the current value. Therefore, some current fuses cannot ensure a reliable protective measure against overcurrent. In this regards, the current fuses may be replaced with an electronic overcurrent detection circuit that performs protective operation against overcurrent when it detects overcurrent in the secondary battery, to thereby protect the secondary battery and electronic devices driven by the battery.
However, the overcurrent detection circuit has a complicated circuit configuration and uses discrete components, so that its power consumption is relatively high. Possibly, this detection circuit may be designed to detect overcurrent in accordance with the output of the current measuring circuit. In this case, however, the measurement accuracy of the current measuring circuit lowers. More specifically, if the overcurrent detection circuit is so constructed as to detect overcurrent in accordance with a current value that is detected by the current measuring circuit, the dynamic range of the measuring circuit must be made wide enough to detect both normal and excessive charging and discharging currents. Thus, not only the detection accuracy of the current measuring circuit but also the overcurrent measurement accuracy thereof are lowered. In the case where the current measuring circuit and the overcurrent detection circuit are configured independently of each other, the circuit configuration is too large-scaled.
As mentioned before, the conventional current measuring circuit and overcurrent detection circuit are comprised of discrete circuit components, and their power consumption is relatively high. If these circuits are incorporated together with the secondary battery in a battery pack, therefore, the battery pack is large-sized, and the limited capacity of the secondary battery is consumed unusefully.
The performance (capacity characteristics) of the secondary battery is very susceptible to temperature, and yet, the battery pack has no substantial room for a heat radiating mechanism or the like. In packaging the battery capacity measuring device together with the secondary battery to realize the battery pack, therefore, heat generation from the measuring device must be minimized.
Since the conventional remaining battery capacity measuring device, comprised of discrete circuit components, consumes high power, it is extremely difficult to realize a battery pack that fulfills the requirement for the reduction of the heat generation. Even though the battery capacity measuring device may be designed to meet certain working conditions of a battery pack and incorporated together with the secondary battery in the battery pack, the measuring device sometimes may fail to operate properly if the working conditions of the battery pack vary. Thus, it is hard to provide a remaining battery capacity measuring device that conforms to various battery pack working conditions.